The present invention relates to the drilling of wells in underground formations. More particularly, the present invention relates to a method and composition for reducing lost circulation of aqueous or oil based drilling fluids.
Drilling fluids, or drilling muds as they are sometimes called, are slurries of clay solids used in the drilling of wells in the earth for the purpose of recovering hydrocarbons and other fluid materials. Drilling fluids have a number of functions, the most important of which are: lubricating the drilling tool and drill pipe which carries the tool, removing formation cuttings from the well, counterbalancing formation pressures to prevent the inflow of gas, oil, or water from permeable rocks which may be encountered at various levels as drilling continues, and holding the cuttings in suspension in the event of a shutdown in the drilling and pumping of the drilling fluid.
For a drilling fluid to perform these functions and allow drilling to continue, the drilling fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts, or in some cases, practically all of the drilling fluid may be lost to the formation. Drilling fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole.
Most wells are drilled with the intent of forming a filter cake of varying thickness on the sides of the borehole. The primary purpose of the filter cake is to reduce the large losses of drilling fluid to the surrounding formation. Unfortunately, formation conditions are frequently encountered which may result in unacceptable losses of drilling fluid to the surrounding formation despite the type of drilling fluid employed and filter cake created.
A variety of different substances have been pumped down boreholes in attempts to reduce large losses of drilling fluids to fractures and the like in the surrounding formations. Different forms of cellulose have been commonly added to drilling fluids to increase their viscosity or resistance to flow into the formation. Also, bulk materials such as almond hulls, walnut hulls, coal particles, dried tumbleweed, paper, coarse and fine mica and even pieces of rubber tires, have been added to drilling fluids in an effort to form bridges or mats over cracks or interstices responsible for drilling fluid escape.
Although lost circulation zones frequently are plugged by such bulk materials, successful, and lasting plugging is not assured. Even if bridges or mats are formed, which sometimes they are not, such bulk materials may deteriorate under high drilling pressures and become less effective.
As alternatives to bulk materials, various slugs of polymers or clays have been used to plug lost circulation zones. Cement, such as Plaster of Paris or a silicate, either alone or in combination with bulk materials, has also been used, particularly when other efforts to plug the lost circulation zone have failed. A primary disadvantage of using cement, however, is that heavier cement particles have a tendency to separate from the aqueous slurry in which they are injected into the borehole and consequently become dehydrated at the formation temperature before the lost circulation zone in the formation can be sealed.
There continues to be a need for alternative effective and efficient methods and materials for reducing or eliminating lost circulation of drilling fluids.